The Wnt/β-catenin pathway regulates diverse biological processes during development and tissue homeostasis through modulating the protein stability of β-catenin (Clevers et al., (2006) Cell 127:469-480; and Logan et al., (2004) Annu. Rev Cell Dev. Biol 20:781-810). In the absence of Wnt signaling, cytoplasmic β-catenin is associated with the β-catenin destruction complex that contains multiple proteins including adenomatous polyposis coli (APC), Axin, and glycogen synthase kinase 3 (GSK3). In this complex, β-catenin is constitutively phosphorylated by GSK3 and degraded by the proteasome pathway. The Wnt signal is transduced across the plasma membrane through two distinct receptors, the serpentine receptor Frizzled, and the single-transmembrane protein LRP5 or LRP6. The Wnt proteins promote the assembly of the Frizzled-LRP5/6 signaling complex, and induce phosphorylation of the cytoplasmic PPPSPxS motifs of LRP5/6 by GSK3 and Casein Kinase I. Phosphorylated LRP5/6 bind to Axin and inactivate the β-catenin degradation complex. Stabilized β-catenin enters the nucleus, binds to the TCF family transcription factors, and turns on transcription.
The large extracellular domain of LRP5/6 contains four YWTD-type β-propeller regions that are each followed by an EGF-like domain, and the LDLR domain. Each propeller region contains six YWTD motifs that form a six-bladed β-propeller structure. Biochemical studies suggest that Wnt proteins physically interact with both Frizzled and LRP6 and induce formation of Frizzled-LRP6 signaling complex (Semenov et al., (2001) Curr. Biol 11, 951-961; and Tamai et al., (2000) Nature 407, 530-535). Besides Wnt proteins, the large extracellular domain of LRP5/6 binds to multiple secreted Wnt modulators, including Wnt antagonist DKK1 and Sclerostin (SOST), and Wnt agonist R-Spondins.
Deregulation of the Wnt signaling pathway has been linked to many human diseases. The Wnt/LRP5/6 signaling pathway plays important roles in tissue homeostasis and regeneration. Wnt signaling promotes bone formation by increasing the growth and differentiation of osteoblasts (Baron et al., (2006) Curr. Top. Dev. Biol 76:103-127). Gain-of-function mutations of LRP5 (Boyden et al., (2002) N. Engl. J Med 346:1513-1521; Little et al., (2002) Am. J Hum. Genet. 70:11-19; Van Wesenbeeck et al., (2003) Am. J Hum. Genet. 72: 763-71) and loss-of-function mutations of Wnt antagonist SOST (Balemans et al., (2001) Hum. Mol Genet. 10:537-543; Brunkow et al., (2001) Am. J Hum. Genet. 68:577-589) both lead to high bone mass diseases. Wnt signaling is also critical for the homeostasis of intestinal epithelium by maintaining the proliferative status of stem cells in the intestinal crypt (Pinto et al., (2005) Biol Cell 97:185-196). Wnt signaling is also critical for kidney repair and regeneration (Lin S L PNAS 107:4194, 2010). In addition, mutations in pathway components such as APC and β-catenin have been associated with human cancers. Recent studies suggest that overexpression of Wnt proteins and/or silencing of Wnt antagonists such as DKK1, WISP and sFRPs promote cancer development and progression (Akiri et al., (2009) Oncogene 28:2163-2172; Bafico et al., (2004) Cancer Cell 6:497-506; Suzuki et al., (2004) Nat Genet. 36:417-422; Taniguchi et al., (2005) Oncogene. 24:7946-7952; Veeck et al., (2006) Oncogene. 25:3479-3488; Zeng et al., (2007) Hum. Pathol. 38:120-133). In addition, Wnt signaling has been implicated for the maintenance of cancer stem cells (Jamieson et al., (2004) Cancer Cell 6:531-533 and Zhao et al., (2007) Cancer Cell 12:528-541).
Accordingly, a need exists for agents that antagonize Wnt signaling at the extracellular level as therapy for diseases associated with aberrant canonical Wnt signaling.